The components in the hot sections of gas turbine engines are susceptible to degradation by hot corrosion attack. Hot corrosion can consume the construction material of turbine engine components at an unpredictably rapid rate, and consequently lead to failure or premature removal of turbine engines. Hot corrosion typically occurs at a temperature range of about 650-950° C.
Molten deposits, such as alkali metal sulfates from intake air or combustion of fuels, are the primary source of hot corrosion. However, other corrosive species such as sulfur dioxide in the environment can accelerate the corrosion attack.
Hot corrosion that is sulfate induced, particularly Type II, has emerged as a concern for engine operation. Many of today's superalloys are more susceptible to Type II corrosion, as they have lower levels of chromium, which as will be explained below, is known to be an effective alloying element in safeguarding against hot corrosion. Additionally, as engine temperature increases, cooler areas of turbine blades, such as in the under platform areas and the surface of internal cooling passages, which were previously operating at temperatures below the onset of hot corrosion, are now becoming exposed to hotter temperature regimes at which Type II hot corrosion can occur. The complicated geometry in these areas can create additional challenges for conventional line-of-sight coating processes such as thermal spray and physical vapor deposition. Rapidly deteriorating air quality in many parts of the world, particularly throughout several countries in Asia, further compounds the problems. Still further, hot corrosion attack often interacts with other degradation modes (i.e., fatigue) during service to accelerate failure of the engine components.
Environmental coatings such as nickel aluminide, platinum aluminide, or MCrAlY overlay coatings are often applied onto the airfoil of gas turbines to enhance oxidation resistance. However, such coatings do not adequately protect engine components against Type II hot corrosion attack.
One method utilized to mitigate hot corrosion attack is the incorporation of chromium onto the surface of a component by a process known as “chromizing”. Two common industrial methods for producing chromizing coatings are pack cementation and vapor phase process.
Pack cementation requires a powder mixture including (a) a metallic source (i.e., donor) of chromium, (b) a vaporizable halide activator, and (c) an inert filler material such as aluminum oxide. Parts to be coated are entirely encased in the pack materials and then enclosed in a sealed chamber or retort. The retort is then heated in a protective atmosphere to a temperature between 1400-2100° F. for 2-10 hours to allow chromium to diffuse into the surface. Although the pack chromizing process has been used since the 1950's, there are several major limitations. First, the pack process generates a large amount of hazardous waste and requires considerable more raw materials than other processes. Second, the pack process is difficult to fully coat selective regions of the parts with complicated geometries, such as the surface of internal cooling passages.
The vapor phase process generally involves placing the parts to be coated into a retort in an out-of-contact relationship with a chromium source and halide activator. The vapor phase process can coat both the external and the internal surfaces of a part, such as a turbine blade having a complicated geometry. However, the chromium content within the resultant coating is generally too low to provide sufficient protection against Type II hot corrosion attack. Furthermore, it is difficult to mask the area where no “chromizing coating” is required. Consequently, the vapor phase process has a tendency to produce a chromizing coating along all surfaces of the part.
Another type of chromizing process is the slurry process described in U.S. Pat. Nos. 4,904,501 and 8,262,812. In the slurry process, a thin layer of aqueous slurry comprising chromium powder and halide activator is directly applied to the substrate surface. The slurry process requires much less raw materials than the pack method, and eliminates the exposure to dust particulates characteristic of the pack method. One of the major limitations of existing slurry processes is that the coating microstructure comprises greater than or equal to 40% by volume alpha chromium (“α-chromium”), which can cause the coating to have poor fatigue crack resistance.
All of the conventional chromizing processes suffer from major drawbacks. First, substantial amounts of oxide and nitride inclusions are formed in the chromizing coating. The inclusions tend to reduce the erosion, fatigue and corrosion resistance of the coating. A second drawback is the formation of a thick and continuous alpha-chromium layer. Although the α-chromium layer offers excellent resistance to type II hot corrosion attack, the α-chromium is brittle and susceptible to thermal fatigue cracking during service. The cracking can propagate into the substrates and lead to the premature failure of the coated system.
In view of the drawbacks of existing chromizing processes there is a need for a new generation chromizing process that can produce a chromium enrich layer with significant reduced level of nitrides, oxides and α-chromium phase, thereby overcoming the current limitations of existing pack, vapor phase and slurry chromizing processes. Furthermore, there is a need for a simple method that can produce a chromizing coating on the selective regions and minimizes masking requirements for areas where “no coatings” are required. There is a need for a method that utilizes considerable fewer raw materials and minimizes exposure of hazardous materials in the workplace. Other advantages and applications of the present invention will become apparent to one of ordinary skill in the art.